Filter
Top Products
DES-7200 Configuration Guide Chapter 2 QoS Configuration
2-2
Carried by the first 3 bits in the Tag Control Information of 802.1Q frame header,
which contains the priority information of one of the 8 categories. These three bits
are generally called User Priority bits.
Carried by the first 3 bits of the ToS field for IPv4 packet header or Traffic Class
field for IPv6 packet header, called IP precedence value; or carried by the first 6
bits of the ToS field for IPv4 packet header or Traffic Class field for IPv6 packet
header, called Differentiated Services Code Point (DSCP) value.
In a DiffServ-compliant network, every device has the same transmission service
policy for the messages with the same classification information, and vice versa. The
class information in the packet can be assigned by all the systems along the way, such
as hosts, devices, or other network devices. It’s based on a policy set by a manager, or
contents in the packet, or both. The assignment of class information in order to identify
packets usually consumes enormous resources of the network device. To reduce the
processing overhead on the backbone network, such assignment is often used on the
network edge. Based on the class information, the devices can provide different
priorities for different traffic, or limit the amount of resources allocated per traffic class,
or appropriately discard the packets of less important, or perform other operations as
appropriate. This behavior of these independent devices is call per-hop behavior in the
DiffServ architecture.
If all devices in the network are providing consistent per-hop behavior, this network
forms the end-to-end QoS solution for the DiffServ architecture.
2.1.2 QoS processing flow
2.1.2.1 Classifying
The process of classifying involves putting the messages to the dataflow indicated with
CoS value according to the trust policy or the analysis of the message contents. As a
result, the core task of classifying is to determine the CoS value of a message. It
happens when the port is receiving the inbound messages. When a port is associated
with a policy-map that represents a QoS policy, the classification will take effect and be
applied on all the messages input through that port.
For general non-IP messages, the switch classifies the messages according to the
following criteria:
If the message itself does not contain any QoS information, which means the
layer-2 packet header has no User Priority bits, it gets the QoS information of the
message by using the default CoS value of the message input port. Like the User
Priority bits of the message, the default CoS value of the port ranges 0~7.
If the message itself contains QoS information, which means the layer-2 packet
header has User Priority bits, it gets the CoS information directly from the
message.